In recent years, from the viewpoint of environmental load reduction, there has been a need for environmentally friendly and clean liquid fuels with a low sulfur content and aromatic hydrocarbon content. From such a viewpoint, as technology which is able to produce a fuel base stock, specifically kerosene or gas oil base stock, not including a sulfur content or aromatic hydrocarbons, that is rich in aliphatic hydrocarbons, methods utilizing the Fischer-Tropsch synthesis reaction (hereunder referred to as “FT synthesis reaction”), in which carbon monoxide gas (CO) and hydrogen gas (H2) are feedstock gases, are being investigated (refer to Patent Document 1 for example).
Conventionally, as a method for producing hydrocarbon oils by the FT synthesis method, a method has been disclosed using a bubble column slurry bed reactor, in which the FT synthesis reaction is performed by blowing a synthesis gas (a mixed gas with CO and H2 as principal components) into a slurry in which solid catalyst particles are suspended within the hydrocarbon oil as a liquid medium (refer to Patent Document 2 for example).
At the time of starting the operation of the bubble column slurry bed reactor (hereunder also simply referred to as “reactor”), that is, at the time of start-up, in general this is performed in a sequence such as the following.
Firstly, the slurry is filled into the reactor. Next, an inert gas such as nitrogen is flowed in from the lower part of the reactor, and the nitrogen drawn out from the upper part of the reactor is returned to the lower part of the reactor, circulating the inert gas. Then, following sufficient flowing of the catalyst in the reactor, in exchange for the nitrogen, the inflow of the synthesis gas from the lower part of the reactor is started. Thereafter, in a state where the synthesis gas is flowed into the reactor, the temperature of the reactor is gradually raised until it reaches a temperature necessary for the FT synthesis reaction, and the FT synthesis reaction is started.
As the hydrocarbon oil used for the slurry at the time of start-up, there is a need to use a high-purity product having suitable flowability that is primarily present as a liquid under the conditions of the FT synthesis reaction, and which has very few impurities, such as sulfur content and aromatic content, so that problems in the catalytic activity in the FT synthesis reaction do not occur.